Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. Typically, a conventional magnetic element is used for storing data in such magnetic memories.
FIG. 1 depicts a conventional magnetic element 10, which may be a conventional magnetic tunneling junction (MTJ) or a conventional spin valve. The conventional magnetic element 10 may be used in a conventional magnetic memory. The conventional MTJ 10 typically resides on a substrate (not shown), uses seed layer(s) 11 and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional barrier layer 16, a conventional free layer 18, and a conventional capping layer 20. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Typically, materials containing Fe, Ni, and/or Co such as FeCo, FeCoB, Permalloy, Co, are used in the conventional pinned layer 14 and the conventional free layer 18. The conventional free layer 18 has a changeable magnetization 19 and may have an easy axis established by a shape anisotropy. The easy axis of the conventional free layer 18 is typically such that the free layer magnetization 19 is parallel (P state) or antiparallel (AP state) with the magnetization 15 of the conventional pinned layer 14. The magnetization 15 of the conventional pinned layer 14 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 12. Although depicted as simple (single) layers, the pinned layer 14 and free layer 18 may include multiple layers. For example, the pinned layer 14 and/or the free layer 18 may be a synthetic layer including ferromagnetic layers antiferromagnetically or ferromagnetically coupled through a thin conductive layer, such as Ru. In such a synthetic layer, multiple layers of CoFeB interleaved with a thin layer of Ru may be used for the conventional pinned layer 14 and/or the conventional free layer 18. Further, other versions of the conventional magnetic element 10 might include an additional pinned layer (not shown) separated from the free layer 18 by an additional nonmagnetic barrier or conductive layer (not shown).
Data, such as a logical “1” or “0”, typically corresponds to the magnetization 19 of the free layer 18 being in the P state or the AP state, respectively. Thus, data are written by setting the free layer 18 in the P state or the AP state. For some conventional magnetic elements 10, this is accomplished by applying an external magnetic field, for example using one or more current-carrying lines (not shown). In other conventional magnetic elements 10 this is accomplished using the spin transfer effect. Reading the state of the conventional free layer 18, and thus the conventional magnetic element 10, is done by measuring the resistance of the conventional magnetic element 10, typically by driving a read current through the conventional magnetic element 10.
To change the magnetization state of the free layer 18 using the spin transfer effect, a current is driven in a current-perpendicular to the plane (CPP) direction (i.e. the z direction in FIG. 1) through the conventional magnetic element 10 having a small enough size. For spin transfer based switching to become important in switching the magnetization state of the conventional magnetic element 10, the lateral dimensions of the magnetic element 10 may be small, for example in the range of a few hundred nanometers or less, in order to facilitate current-based switching through the spin transfer effect.
The write current used in switching the conventional magnetic element 10 via spin transfer is typically a bidirectional write current. For a bidirectional write current, a write current is applied one way (from the conventional pinned layer 14 to the conventional free layer 18) to switch the conventional magnetic element 10 to the AP state, while the write current is applied in the opposite direction to switch the conventional magnetic element 10 to the P state. Note that these write currents may have different magnitudes. When electrons travel through the conventional pinned layer 14, they become spin-polarized, with electron spins preferentially pointing along the magnetization 15 of the conventional pinned layer 14. For current driven from the conventional free layer 18 to the conventional pinned layer 14, the electrons polarized by the conventional pinned layer 14 enter the free layer 18 and exert a torque on the magnetization 19, which can cause generation of spin waves or even complete switching of the magnetization 19 to the P state. When switching to the AP state, electrons having their spins aligned antiparallel to the magnetization 15 of the conventional pinned layer 14 are more likely to reflect back to the conventional free layer 18. These electrons may exert a torque on the magnetization 19 and may cause generation of spin waves or complete switching of the magnetization 19 to the AP state.
A measure of the current density in the device for observing the switching is given by on-axis magnetization instability current density. For a monodomain small particle under the influence of spin transfer torque, this instability current density, or critical switching current may be given by:
            J      c0        =                  2        ⁢        e        ⁢                                  ⁢        α        ⁢                                  ⁢                  M          S                ⁢                              t            F                    ⁡                      (                          H              +                              H                K                            +                                                H                  d                                2                                      )                                      ℏ        ⁢                                  ⁢        η              ,
where e is electron charge, α is the Gilbert damping constant, MS is the saturation magnetization, tF is the thickness of the free layer, H is the applied field, HK is the effective uniaxial anisotropy of the free layer (including shape and intrinsic anisotropy contributions), Hd is the out-of-plane demagnetizing field (typically equal to 4πMs for a thin ferromagnetic film), h is the reduced Planck's constant, and η is the spin transfer efficiency related to polarization factor of the incident current. At this current density the initial position of the free layer magnetization 19 along the easy axis becomes unstable and it starts precessing around the easy axis. As the current is increased further, the amplitude of this precession increases until the magnetic element 10 is switched into the other state. For fast switching of the free layer magnetization 19, in nanosecond regime, the required current is several times greater than the instability current Jc0.
Although the bidirectional write current can switch the magnetization 19 of the conventional free layer, its use may have drawbacks. For example, significant limitations may be imposed on the maximum allowed switching current to be passed through the conventional magnetic element 10. In particular, when used in a memory, the conventional magnetic element 10 is used in conjunction with a selection transistor. The bidirectional current is limited by the size of the selection transistor (not shown). Several techniques and material optimization have been performed to decrease this current. However, further improvements are still desired.
Accordingly, what is needed is a method and system that may improve performance of the conventional magnetic element 10 when current-based switching is employed. The method and system address such a need.